FIG. 1 is a circuit configuration diagram showing an example of a conventional power converter. In FIG. 1, a commercial power supply 1 (50 Hz or 60 Hz, AC 80 V to 260 V) and a liquid crystal television (TV) system 2i are provided. The liquid crystal TV system 2i includes a first direct current (DC) converter 3′, a second DC converter 4′, a third DC converter 5′, a backlight (B/L) 6 having electric discharge tubes 60a and 60b, a liquid crystal driver 8, an image processing circuit 9, a speaker 10 and a direct current-alternating current (DC-AC) converter 15 having a leakage transformer.
The first DC converter 3′ is configured to convert AC voltage from the commercial power supply 1, into DC voltage (DC 380 V, for example), and also to correct the power factor. The second DC converter 4′ is a main power supply, and is configured to isolate the primary side and the secondary side from each other, and to convert the DC voltage from the first DC converter 3′, into predetermined DC voltage (DC 24 V, for example). The DC-AC converter 15 is configured to convert the DC voltage into AC voltage (65 kHz, AC 1500 Vrms, for example), and to thereby light the electric discharge tubes 60a and 60b. 
The second DC converter 4′ is configured to supply the predetermined DC voltage to the liquid crystal driver 8 to drive the liquid crystal driver 8. The third DC converter 5′ is configured to electrically isolate the first DC converter 3′ from the image processing circuit 9 and the speaker 10, and also to convert the DC voltage from the first DC converter 3′, into DC 12 V and DC 36 V and then supply DC 12 V and DC 36V respectively to the image processing circuit 9 and the speaker 10 to drive the image processing circuit 9 and the speaker 10.
Thus, the power converter shown in FIG. 1 is capable of causing the electric discharge tubes 60a and 60b to emit light by converting the AC power (voltage) from the commercial power supply 1 into high-voltage and high-frequency AC power (voltage).
As techniques of such a conventional kind of power converter, those described in Japanese Patent Application Publications Nos. 2005-71681 and Hei 10-50489, U.S. Pat. No. 5,930,121 (the second paragraph in the Detailed Description of the Invention), and U.S. Pat. No. 5,615,093 (FIG. 3) are known, for example.
However, in the power converter shown in FIG. 1, a power conversion is performed three times in total, that is, power conversions in the first DC converter 3′, the second DC converter 4′, the DC-AC converter 15, during power transfer from the commercial power supply 1 to the B/L 6 which includes the electric discharge tubes 60a and 60b, and consumes the largest load power.
Methods for reducing power consumption (save energy) in an LCD-TV are, for example, to enhance the luminance efficiency of a light source itself and to enhance the power conversion efficiency of each power conversion block. In addition to these methods, another effective method is to reduce the number of power conversions to be performed before the power reaches a light source requiring the greatest power.
While a light-emitting diode (LED) can be lit with DC voltage, application voltage (drive voltage) of the LED is determined on the basis of the IF-VF characteristics and the temperature characteristics of the LED. For this reason, when the LED is controlled to emit light with constant luminance (constant current is supplied), basically, the drive voltage of the LED cannot be used directly as input voltage of another load circuit since some variations occur in the drive voltage. In addition, in the case of home appliances, such as TVs, which people can easily touch, the commercial power supply 1 and the B/L 6 need to be electrically isolated from each other for safety.
In the case of the power converter shown in FIG. 1, it is also conceivable to omit the second DC converter 4′ and input an output of the first DC converter 3′ directly to the DC-AC converter 15, for example. In this case, however, isolation between the primary side and the secondary side is made at the leakage transformer in the DC-AC converter 15 where input and output voltages both are high. This leads to problems of an increase in the price of the transformer and causing eddy current loss at a conductive pattern of a peripheral printed circuit board (PCB) due to high leakage flux from the transformer. For this reason, it is more ideal if isolation between the primary side and the secondary side is made in one of the DC converters.